Vascular calcification can be characterized by formation of very small, dispersed crystals of hydroxyapatite (HA) and as large calcified deposits in vascular tissue, such as arteries. (Amann, K. Clin J Am Soc Nephrol 2008, 3, 1599-605). Extracellular pyrophosphate (PPi) is a key endogenous inhibitor of vascular calcification by inhibiting HA formation. (Lomashvili, K. A. et al., J Am Soc Nephrol 2004, 15, 1392-1401; Fleisch, H. et al., Nature 1966, 212, 901-903).
Ectonucleotide pyrophosphatase pyrophosphorylase (NPP1) is an ectoenzyme that cleaves ATP to produce extracellular pyrophosphate (PPi). Pyrophosphate is a potent inhibitor of hydroxyapatite formation and, under normal conditions, functions to inhibit vascular calcification.
Deficiency of NPP1 in humans results in reduced circulating PPi levels and has been implicated in conditions such as arterial calcification and generalized arterial calcification of infancy (GACI). (Rutsch, F. et al., Am J Pathol 2001, 158, 543-554). When fed a high-phosphate diet, mice lacking NPP1 (Enpp1−/−) also have reduced PPi levels and exhibit a similar phenotype as NPP1 deficient humans. (Harmey, D. et al., Am J Pathol 2004, 164, 1199-1209). Vascular calcification is also a well-recognized and common complication in chronic kidney disease (CKD) and end-stage renal disease (ESRD) subjects, and is associated with increased morbidity and mortality. (Giachelli, C. J Am Soc Nephrol 2004, 15, 2959-64; Raggi, P. et al., J Am Coll Cardiol 2002, 39, 695-701).
Ectonucleotide pyrophosphatase/phosphodiesterase 1 (NPP1/ENPP1/PC-1) deficiency is a rare disease caused by mutations in NPP1, a type II transmembrane glycoprotein. NPP1 cleaves a variety of substrates, including phosphodiester bonds of nucleotides and nucleotide sugars and pyrophosphate bonds of nucleotides and nucleotide sugars. NPP1 deficiency has been associated with idiopathic infantile arterial calcification (IIAC), insulin resistance, hypophosphatemic rickets, and ossification of the posterior longitudinal ligament of the spine.
IIAC, a rare autosomal recessive and nearly always fatal disorder, is characterized by calcification of the internal elastic lamina of muscular arteries and stenosis due to myointimal proliferation. There are more than 160 cases of IIAC that have been reported world-wide. The symptoms of the disease most often appear by early infancy, and the disease is lethal by 6 months of age, generally because of ischemic cardiomyopathy, and other complications of obstructive arteriopathy including renal artery stenosis.
Although defects in the NPP1 protein have been implicated in such serious disease as IIAC, currently no treatment is available for those who are affected by the disease and other calcification diseases caused by high total body burden of calcium and phosphorus due to abnormal bone metabolism; low levels of circulating and locally produced inhibitors of phosphate producers; or impaired renal excretion.
Current therapeutic options to prevent vascular calcification have limited efficacy and undesirable and/or unacceptable side effects. For example, very large quantities of exogenous PPi are needed for efficacy and other inhibitors hydroxyapatite formation inhibit calcification of bone and can lead to osteomalacia. In particular, direct administration of exogenous PPi was found to prevent calcification in uremic animal models. (O'Neil, W. C. et al., Kidney Int 2011, 79, 512-517; Riser, B. L. et al., Nephrol Dial Transp 2011, 26, 3349-3357). But, this approach required high doses of PPi, due to the short half-life of PPi, and resulted in supraphysiologic plasma levels of PPi leading to local irritation. Bisphosphonates, which are non-hydrolyzable analogs of PPi, have been used to treat vascular calcification, e.g., in animal models. (Fleisch, H. et al., Europ J Clin Invest 1970, 1, 12-18; Price, P. A. et al., Arteriosclerosis Throm and Vas Bio 2001, 21, 817-824; Price, P. A. et al., Kidney Int 2006, 70, 1577-1583; Lomashvili, K. A. et al., Kidney Int 2009, 75, 617-625). However, bisphosphonates also inhibit bone formation. Bisphosphonates can delay but not stop calcification in subjects with GACI (Rutsch, F. et al., Circ Cardiovasc Genet 2008, 1, 133-140), and, as in animals, lead to osteomalacia. (Otero, J. E., et al., J Bone Miner Res 2013, 28, 419-430).
Braddock, D. et al., (WO 2014/126965A2) discloses compositions and methods for treating pathological calcification and ossification by administering NPP1. Quinn, A. et al., (WO 2012/125182A1) discloses a NPP1 fusion protein to treat conditions including GACI, arterial calcification, insulin resistance, hypophasphatemic rickets, and ossificaiton of the posterior longitudinal ligament of the spine.
In spite of considerable research in the field, there is a continuing need for new therapies to effectively inhibit vascular calicification, preferably without causing osteomalacia. There is also a need for an effective and safe medicament for the treatment of IIAC, vascular calcification in chronic kidney disease (VCCKD), pseudoxanthoma elasticum (PXE), insulin resistance, hypophosphatemic rickets, and ossification of the posterior longitudinal ligament of the spine.